H-Bridge: Black Box or Are Details Important?

Engineers in all disciplines use electronics in their designs for sensing, actuation, and real-time control. Today there are few exceptions. A common component is an operational amplifier (op-amp), which most engineers treat as a black box containing many transistors and resistors. Its performance is primarily determined by the components (e.g., resistors and capacitors) surrounding it as long as the op-amp has negative feedback and its limitations are not exceeded.

Another common electronic component is the H-bridge. Any engineer who has ever controlled a motor has most likely used the H-bridge, but perhaps treated it as a black box with no thought as to how it works and how it affects overall system performance. The H-bridge needs to be thoroughly understood for model-based design and optimum system performance.

The H-bridge, as shown in the diagram, is named because of its configuration. It has four switching elements (transistors, MOSFETs are a good selection, p-channel for high side, and n-channel for low side) with the load (usually brushed DC or step motor) at the center. The diodes are of the Schottky type with short turn-on delay. The four transistors can be turned on and off independently. If transistors 2 and 3 are turned on, the motor turns in one direction. Turn transistors 1 and 4 on and the motor turns in the opposite direction. The transistors are usually controlled in a pulse-width modulated (PWM) fashion. When the transistor is on, it behaves like a small temperature-dependent resistor -- the lower the value the better for heat dissipation.

When the transistor is completely off, it conducts no current. MOSFETs are voltage-driven devices. The gate forms a parasitic capacitor with the source, and this capacitance limits the speed at which the transistor can be turned on and off. In the transitional periods, the power dissipation due to switching is significant, especially when the switching frequency is higher than a few hundred hertz (Hz). The role of the diodes is often overlooked and they are intrinsic in MOSFETs. While the bridge is on, two of the four transistors carry the current and the diodes have no role.

However, once the bridge is turned off, the transistors will not conduct current. When the load is inductive, as with motors, the electromagnetic field associated with it will collapse when the transistor is turned off and the diodes provide a low-resistance path for that current to flow and thus keep the voltage on the motor terminals within a reasonable range. The dissipated heat from the diodes can be of the same order of magnitude as the heat dissipation from the transistor switching.

The load (motor) is modeled as an inductor (Lm), resistance (Rm), and speed-dependent voltage (back emf) in series, the values of which are all dependent on motor rotor position. The motor torque is proportional to the current flowing through this series combination. There are two extremes. When the motor runs with no load, the current is low and the motor terminal voltage is close to the back-emf voltage. When the motor is stalled, the back-emf voltage is zero and the motor acts like an inductor.

The H-bridge can be driven in many different ways. In general, the on-time behavior is rather simple: Turn on one high-side transistor and the opposite low-side transistor to allow current to flow through the motor. It is the off-time drive that makes the difference. Since transistors 1 and 2 (or 3 and 4) should never be turned on at the same time, there are only three different combinations for those two switches: Transistor 1 conducts, or transistor 2 conducts, or neither conducts. There are many different drive modes. Andras Tantos has provided an excellent, detailed explanation, and I highly recommend it.

A black box approach to some commonly used devices is justified, but the H-bridge is not one of them.

dgrieg, to produce good circuits using text we used to use DOS EDIT. the closest one can get now os MS notpad, which works fairly well. But I produced a lot of cable diagrams using edit and "wordstar" non-document. Back in the olden days. Those were the days my friend.

WilliamK, the value of Bipolar over MOSFET's or vice versa is a very convoluted relationship between voltage, current and switching speed as you well know. Bipolars have a VCEsat figure that can be markedly lower than the RDSon of a FET when they're hi voltage devices. Then there's the to SOA foldback behaviour of bipolars compared to the straight line SOA with MOSFET's which is really handy if there's a lot of load in the inductance so having the bipolar/MOSFET split per your description may make real good sense for some applications. I think a heatsink (or rather lack of need for one) could very easily make up for the added cost of drive circuitry.

That said, SOA would be the one thing that I would be keeping a real close eye on.

Interesting side note, the authour introduced us to the black box concept by talking about Opamps, I would have thought that they too should never be black-boxed until after the details are nutted out.

You can do anything with software. However, it does not run with out power. And when you violate the laws of physics there will be smoke and perhaps fire. We had a software engineer who did not bother to read the rise time rateing advised. As a ressult the circuit bounced when good deivices were used. The orginal fix to the fast software was slow devices with little gain. The fix was do change the software.

The H-bridge as shown will be about 90% efficient because half of the MOSFETs are hard-switched. This is a lot of electronics loss in a multi-kilowatt-level motor. Soft switching circuits can be added to the basic bridge to bring the efficiency to over 95% for AC or DC motor drives, maybe to 98%. Soft switching has the additional benefit of being less stressful on the MOSFETs and the motor windings, so it increases reliability.

I have seen an alternative arrangement for H-bridges that used to mosfets as the upper transistors and two NPN devices at the lower side. The benefit that was claimed was that the upper sidedevices had the lower voltage drop while conducting while the NPN devices worked faster for the PWM control function. And the shunting diodes could be external to the transistors so that their conduction loss during switching would not affect the transistor dissipation. Aside from making the drive circuit a bit more complex it certainly sounded like a worthwhile option.

Is there a downside that I have missed? Aside from not fitting into a single IC package?

Actually there are two H-bridge configurations, this is one of them. The other is made of all N-channel devices, the idea is that Rdson is always higher for P-channel devices, so by using all N-channel with a heavy load you will tend to lower losses. This is the more common configuration when an IC with a high level of integration is used to drive the power devices. Since such a configuration requires a drive voltage that is 5 to 10 volts higher than the supply voltage, the IC must include a charge pump, and it is by providing same that the IC "earns its keep" (merits the possibly higher cost in the application circuit). Such designs don't generally require additional analysis however, since presumably the design of the switchover time provided by the IC "guarantees" that cross-conduction in the bridge cannot occur.

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